Power metal oxide semiconductor field effect transistors (MOSFETs), while rugged in their ability to withstand current surges, are particularly sensitive to damage caused by voltage transients. The transient protection circuits typically used to protect the MOSFETs from high voltage spikes conventionally use resistance-capacitor (RC) snubber circuits. RC snubber circuits tends to be "power hungry", and utilize capacitors of rather large magnitude thereby making the devices relatively expensive and impractical in a modern, efficient power amplifier circuit.
The present invention is directed to a voltage transient protection circuit for MOSFETs that avoids the need for RC snubber circuits. The present invention utilizes a transient protection circuit that is connected between two MOSFET drain terminals and the output transformer primary winding of a push-pull output circuit. During operation a relatively small magnitude capacitor is raised to a direct current voltage level equal to approximately twice the applied circuit voltage. This arrangement is effectively utilized to damp out transients by limiting the transient level to a voltage of approximately five percent over the voltage of twice the source voltage. This level of voltage is not harmful and can be readily tolerated by the MOSFETs.
In accordance with the present invention, there is provided a voltage transient protection circuit for a pair of metal oxide semiconductor field effect transistors (MOSFETs) operated in a push-pull circuit with said MOSFETs each having a drain connection, a source connection, and a gate connection, including: a transformer having a tapped primary winding and a pair of primary end windings; a first of said pair of end windings connected to a first of said drain connections, and a second of said pair of end windings connected to a second of said drain connections; a pair of diodes with each of said diodes connected to a separate transformer end winding with said diodes further connected in common to a first end of an energy dissipation means; said energy dissipation means having a second end connected to said transformer tapped winding; said transformer tapped winding further connected by circuit means to a source of potential for said MOSFETs and with said source of potential further including a circuit ground; and both of said MOSFETs source connections being connected to said circuit ground; said diodes and said energy dissipation means limiting voltage transients in said protection circuit to a level that is not harmful to said MOSFETs.